Deepest hydrozoan

A hydrozoan specimen provisionally identified as the rhopalonematid trachymedusa Pectis profundicola was filmed at a depth of 10,063 m (33,015 ft) in the north-west Pacific’s Philippine Trench in March 2021, as part of the “Ring of Fire” scientific expedition. The findings were reported in a comprehensive study into cnidarian extreme depths in all five oceans based on hundreds of observations that took place between 2021 and 2022. The study was published in the journal Marine Biology in March 2023.

Hydrozoans (class Hydrozoa) are closely related to jellyfish and corals, all of which are part of the phylum Cnidaria. With approximately 3,700 species, hydrozoans can be both solitary organisms or colonial, a single body formed of hundreds of individuals (such as the surface-dwelling Portuguese man o’ war).

Despite sharing many of the same anatomical traits and behaviours, “true jellyfish” are currently taxonomically treated as a separate class: Scyphozoa (the scyphozoans).

The revelation of a new deeper extent for this hydrozoan appeared in the study Maximum depth extensions for Hydrozoa, Tunicata and Ctenophora. Its authors are deep-sea marine biologists Alan J Jamieson, Dhugal J Lindsay and Hiroshi Kitazato.